This invention relates to the deposition of conformal aluminum films onto a refractory metal nitride layer, and more particularly to an improved deposition process in which pretreatment of the refractory metal nitride layer prior to deposition of the aluminum results in the production of a smooth film and essentially void free aluminum plug.
In the fabrication of integrated circuits, electrical contact must be made to isolated active device regions formed within a semiconductor wafer/substrate. The active device regions are connected by highly electrically conductive paths or lines which are fabricated above an insulator material which covers the substrate surface. To provide electrical connections between the conductive path and active device regions, an opening in the layer of insulator material is provided to enable the electrically conductive films to contact the desired regions. Such openings are typically referred to as contact openings or simply contacts. Ultimately, an electrically-conductive filling material such as tungsten metal would be provided in the contact opening for making electrical connections between the surface conductive paths and the active device regions on the substrate.
As transistor active area dimensions have approached less than one-half micron in diameter on ultra large scale integrated circuit (ULSI) devices, conventional process techniques have proved to be unable to meet the needs of providing low resistance paths to the contacts. One of the more difficult problems in semiconductor device fabrication has been the filling of via or contact holes that have only submicron diameters and high aspect ratios (i.e., high length to diameter ratios) of 4:1 or greater with metal. Sputtering techniques do not provide good step coverage at such high aspect ratio holes.
Chemical vapor deposition techniques do provide better results in filling such holes. Tungsten metal has been used as the plug material to fill high aspect ratio contact holes and vias in semiconductor device manufacture. However, tungsten, and the processes used to deposit the tungsten metal, have several drawbacks. Current chemical vapor deposition techniques for tungsten (W) use a tungsten fluoride gas (such as WF.sub.6). The fluoride ions in the gas will attack the silicon surface of a semiconductor substrate. The tungsten metal itself may react with the silicon substrate. Additionally, while tungsten has reasonably low electrical resistivity, there are other metals with even lower resistivities which could provide a better electrical contact and flow path. Chemical vapor deposition of aluminum has become increasingly important in the manufacture of semiconductor devices. Aluminum has always been used as a conductor material because of its properties of high conductivity, an electrical resistivity less than that of tungsten, high adherence, and low stress. Aluminum is thus a desirable candidate to serve as the electrically conductive material to fill the contact holes. However, aluminum films deposited using chemical vapor deposition (CVD) techniques have heretofore had rough surfaces, voids, and have not been conformal without further surface modification. Such surface roughness may lead to problems with later photolithography steps resulting in a decrease in resolution.
Attempts have been made to improve the quality of CVD aluminum films. It is believed that substrate pretreatment using a titanium-containing precursor improves the surface morphology of aluminum films by providing nucleation sites. One prior process uses chemical vapor deposition of TiCl.sub.4 vapor to pretreat wafer substrates prior to aluminum deposition. The TiCl.sub.4 molecules adsorb onto the substrate surface. However, residual chlorine at the interface between the substrate and aluminum has been known to cause corrosion of the aluminum metal.
More recently, metalorganic titanium compounds have been used to deposit titanium-containing species directly onto silicon and silicon dioxide surfaces as a pretreatment for later CVD of aluminum. See, Sugai et al, "Aluminum Chemical Vapor Deposition with New Gas Phase Pretreatment Using Tetrakis dimethylamino-titanium for Ultralarge-scale Integrated-Circuit Metallization", J. Vac. Sci. Technol. B 13(5), September/October 1995, pp. 2115-2118. However, again, problems may arise as the directly-deposited aluminum metal may attack the silicon and silicon dioxide surfaces.
Accordingly, the need still exists in this art for a technique and process of depositing a smooth and conformal layer of aluminum to produce a void free film in via holes and contacts for integrated circuits.